The overall objective of this proposal is to develop mouse models which can be used to efficiently test vaccine or gene therapy strategies for the prevention or treatment of HIV infection. The long-term goal is the extrapolation of findings from such animal experimentation to clinical utilization. A two-step innovative approach will (1) develop and characterize immunodeficient stocks of transgenic NOD/LtSz-scid mice expressing human major histocompatibility complex (MHC) class I or class II molecules and (2) assess the ability of these mice to support engraftment, development, and immunological function of MHC-matched human lymphohematopoietic cells. Although NOD/LtSz-scid mice support improved engraftment with human lymphoid cells and hematopoietic progenitor cells compared with previously existing models, the long-term function and survival of transplanted human lymphoid cells in these mice remains limiting. Furthermore, human hematopoietic progenitor cells fail to develop in the thymus of these engrafted mice. We hypothesize that expression of human MHC class I and class II transgenes in NOD/LtSz-scid mice will facilitate murine host antigen presentation to MHC-matched human CD8+ and CD4+ T cells and will support survival of these engrafted cells in the periphery. We further hypothesize that expression of human MHC class I and class II molecules on thymic stromal cells will support intrathymic differentiation of engrafted human hematopoietic progenitor cells into CD8+ and CD4+ T cells. Specific Aims 1 and 2 will utilize classical mammalian genetics in combination with transgenic and knockout technology to develop NOD/LtSz-scid mice that express genomic constructs of human MHC class I or MHC class II molecules, respectively, in the absence of classical mouse MHC class I or class II molecules. Specific Aim 3 will assess expression of the human MHC transgenes and will determine the phenotypic effects of these transgenes on the immunological and hematological systems of NOD/LtSz-scid mice. The final Specific Aim will (a) evaluate the abilities of the human MHC class I and class II transgenic mice to support engraftment and immunological function of human lymphoid cells and will (b) assess the abilities of these mice to support engraftment of human hematopoietic progenitor cells and the intrathymic differentiation of these cells into mature T cells. Our proposed integrated approach between investigators at The Jackson Laboratory and The University of Massachusetts Medical School will permit the development, characterization, and validation of these small animal models as hosts for the development and function of human lymphoid cells. The experimental findings will establish the potential utility of these models for the testing of experimental HIV vaccines and for HIV gene therapy.